RF filters can be evaluated using certain Figures of Merit (FOM). One such FOM is the quality factor (Q) and another such FOM is the effective coupling coefficient (K2). Q is a measure of a material's mechanical losses and is directly related to filter insertion loss. Q is a function of the identity and quality of the piezoelectric material as well as the effectiveness of isolation between the piezoelectric medium and the substrate. K2 is a measure of the effectiveness of piezoelectric coupling and is critical in determining a filter's bandwidth.
Epitaxy, epitaxial growth, and epitaxial deposition refer to growth or deposition of a crystalline layer on a crystalline substrate. The crystalline layer is referred to as an epitaxial layer. The crystalline substrate acts as a template and determines the orientation and lattice spacing of the crystalline layer. The crystalline layer can be, in some examples, lattice matched or lattice coincident. A lattice matched crystalline layer can have the same or a very similar lattice spacing as the top surface of the crystalline substrate. A lattice coincident crystalline layer can have a lattice spacing that is an integer multiple of the lattice spacing of the crystalline substrate. The quality of the epitaxy is based in part on the degree of crystallinity of the crystalline layer. Practically, a high quality epitaxial layer will be a single crystal with minimal defects and few or no grain boundaries. Traditionally, metal contact layers are applied to an epitaxial structure at some point in the upstream processing. With today's complex epitaxial structures often incorporating more than one device functionality, this can require extensive etching and deposition of metals on wafers with a large amount of topography.